1. Field of Invention
The present invention relates to power converters, and more particularly, relates to synchronous rectifying for soft switching power converters.
2. Description of Prior Art
FIG. 1 shows a circuit schematic of a conventional soft switching power converter 1. It includes a transformer 10 to provide isolation from line input VIN to the output VO of the power converter 1 for safety. Switches 20 and 30 develop a half bridge circuit to switch a resonant tank and the transformer 10. An inductor 5 and a capacitor 40 form the resonant tank. The inductor 5 can be an inductance device or the leakage inductance of a primary winding NP of the transformer 10. The inductance L of the inductor 5 and the capacitance C of the capacitor 40 determine the resonance frequency f0 of the resonant tank.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The transformer 10 transfers the energy from the primary winding NP to the secondary windings NS1, NS2 of the transformer 10. Rectifiers 61, 62 and a capacitor 65 rectify and filter voltages at the secondary windings NS1, NS2 to generate a DC voltage VO at the output of the power converter 1. The detail operation and description of the soft switching power converter 1 can be found in text book “Resonant Power Converters” by Marian K. Kazimierczuk and Dariusz Czarkowski, 1995 by John Wiley & Sons, Inc.
Although the soft switching of the power converter can achieve high efficiency and low EMI (electric-magnetic interference) performance, the forward voltage of rectifiers 61 and 62 still causes significant power losses. Using transistors as the synchronous rectifiers is a higher efficiency approach, such as “Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter” disclosed by Yang, in U.S. Pat. No. 7,173,835. However, the disadvantage of this prior art is an additional power consumptions caused by saturable inductors, etc. Besides, it is not optimized for soft-switching topologies. The object of present invention is to provide the synchronous rectifying for soft switching power converter to achieve higher efficiency.